Root crop diseases

by Sanne Kure-Jensen

There are three main causes of root crop diseases: fungi, nematodes and bacteria. Professor Rob Wick of the Stockbridge School of Agriculture offered tips on preventing, diagnosing and mitigating root crops diseases in his workshop “Root Crop Diseases: from top to bottom” at the 2017 New England Vegetable & Fruit Conference.

Pythium and Rhizoctonia are common fungi present in all soils while Sclerotinia, Thielaviopsis and Itersonilia are less common. Rhizoctonia can cause black lesion on carrots or parsnips.

Sclerotinia rot affects a large host range of crops. Affected storage carrots have white mold spots. Sclerotinia can linger in soils for 10 years. Fungal structures called sclerotia harden off as rigid black structures to survive winter and reactivate the next season. Sclerotinia is more active during cool (50-70°F) moist weather; the soil needs to be very moist for five to seven days before the sclerotia produce tiny mushrooms that release spores. Effective crop rotations are corn and small grains that are not at risk for infection by Sclerotinia.

Thielaviopsis basicola leads to gray to black lesions on carrots. Soil fungal levels will increase each year the same crop family is grown in the same field. The best protection is annual rotation with non-susceptible crops.

Nematodes are difficult to manage and can cause root knots, cysts or lesions. Root-knot nematodes are often (inadvertently) spread between sites by moving contaminated soil. Growers should take care not to move soil known to harbor root-knot nematodes to non-contaminated soil. Tractors and equipment can be hosed down with water.

Itersonilia is also known as sole head or canker and causes dark lesions on parsnip roots. Root knot nematodes cause forking of roots and may make carrots or parsnips unsalable.

Common bacterial problems include Streptomyces, which can lead to the formation of raised scabs on carrots. Growers can lower soil pH to reduce risk of scab and should avoid planting root crops right after potatoes.

Wick said the best management for soil-borne diseases is to avoid growing any at-risk crop in affected fields. Long cycle crop rotation could provide income with alternate crops until soil-borne fungal problems diminish. Improving drainage with a chisel plow can help minimize soil-borne diseases.

Long cycle crop rotation could provide income with alternate crops until soil-borne fungal problems diminish.

The foliage of root crops is also vulnerable to fungal and bacterial diseases. When root tops are weak, mechanical harvesting becomes nearly impossible. Fungal pathogens like Alternaria and Colletotrichum affect carrots. Ascochyta affects parsnips. Cercospora affects fresh market beets and downy mildew damages beet tops — either could make the tops unsalable. Bacterial diseases like Xanthomonas campestris impact carrots and other vegetable crops.

Wick said the best management for foliar diseases is to hope for pathogen free seed and disease resistant cultivars. Be sure to rotate at least two years away from carrots and parsnips if Alternaria, Septoria or Colletotrichum are suspected. Be sure to rotate at least two years away from beets if Cercospora is suspected. To plan fungicide applications, Wick recommended growers use TomCast through the Northeast Weather Association (NEWA) https://nysipm.cornell.edu/newa.

Wick strongly recommended plowing under plant debris after harvest to reduce risk of future fungal diseases of the foliage.

Wick recommended all growers reference the latest “New England Vegetable Management Guide” for current production and pest management techniques.

Email questions to Professor Rob Wick at rlwick@umass.edu or call 413.545.1045.

Learn more at newenglandvfc.org.

2018-06-01T11:16:47+00:00June 1st, 2018|Grower East|0 Comments

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